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Binary Recap Color & Images

CS110: Computer Science and the Internet. Binary Recap Color & Images. Homework 2 http :// cs.wellesley.edu /~cs110/assignments/ assignmentPageLayout / index.shtml Clarification Project http:// cs.wellesley.edu /~cs110/students/ accounts.html. Binary is Magic!. ASCII code.

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Binary Recap Color & Images

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  1. CS110: Computer Science and the Internet Binary RecapColor & Images

  2. Homework 2 http://cs.wellesley.edu/~cs110/assignments/assignmentPageLayout/index.shtml • Clarification • Project • http://cs.wellesley.edu/~cs110/students/accounts.html

  3. Binary is Magic!

  4. ASCII code ASCII code allows 256 “characters” to be represented UTF-8 encodes more than 1 million characters from the Unicode character set Line-endings

  5. Scott’s exercise

  6. Standard Colors All standards-compliant browsers should handle these color names These color names can be used with the CSS properties of color and background-color: For other colors, it’s safest to use a numericalrepresentation…

  7. RGB Color Human retina: 3 kinds of color-sensitive photoreceptors, traditionally called red, green, and blue cones Visible colors can be created by adding different amounts of the three primary colors, red, green, and blue Color monitors display colors by adding different amounts of red, green, and blue light RGB color components are usually defined on a scale from 0 to 255 Over 16 million colors can be represented this way! Two ways to specify turquoise using rgb in CSS: color: rgb(64,224,208); color: rgb(25%,88%,82%); Try this game: http://www.scottkim.com/newmedia/colormergb.html

  8. Hexadecimal Notation Numbers in the range 0-255 are represented in base 16, using two “digits” that each have 16 possible values, drawn from 0..9, A..F decimal hexadecimal • Converting decimal to hexadecimal: • Left digit: divide by 16, convert quotient to 0..9,A..F • Right digit: convert remainder to 0..9,A..F • Converting hexadecimal to decimal: • Convert left digit to 0..15, multiply by 16 • Convert right digit to 0..15, add to (1) • Example: A716 16710 • A16 is 1010, 10*16 = 160 • 716 = 710 160+7 = 167 • Example: 20110 C916 • 201/16 = 1210 C16 • remainder is 910 916

  9. Hexadecimal Colors # 94 00 D3 RGB Using hexadecimal colors in CSS: blockquote { color: #9400D3; background-color: #E6E6FA; }

  10. Is there a website we can use to input color codes and see what they are? • http://html-color-codes.info

  11. 38 pixels high 44 pixels wide Digital Images Images are often represented in a computer as a 2-dimensional (2D) array of pixels

  12. Image Representation RGB color for each pixel is stored in 24 bits (3 bytes) How much memory is needed to store a 300x500 pixel image? • Problem: large image files can take a long time to download! • Example: On a 300 kbps (300k bits per second) cable modem, a 450 kB image takes about 12 seconds to download • Solutions: • Downsampleimages to make them smaller (fewer pixels) • Compress the images, using “lossless” compression (no information is lost) or “lossy” compression (less important information is lost) techniques

  13. Indexed Color

  14. Indexed Color image image contents color palette • How large is a file that stores a 300x500 pixel indexed color image with 4 colors, and its color palette? • 300x500 pixels, with 2 bits per pixel (why?) • 300 x 500 x 2 = 300,000 bits • (2) 4 colors, with 24 bits per color • 4 x 24 = 96 bits •  ~37.5kB (compared to the 450kB uncompressed file!) 2 bits per pixel 24 bits per color

  15. Indexed Color • How large is a file that stores a 300x500 pixel indexed color image with 16 colors, and its color palette? • What is the number of bits used to store the image pixels? • (2) What is the number of bits used to store the color palette? • Total file size (in bytes): “bits per pixel” is also called the “bit depth” bit depth must be large enough to store all the colors

  16. GIF Indexed Color The GIF file format is an indexed color format that allows 256 colors (out of a possible 16 million colors!) • Computing the file size for a GIF image, in bytes: • number of bytes to store the image pixels: • (width * height * bit-depth) / 8 • (2) number of bytes to store the color palette: num_colors * 3 • (3) total number of bytes*: • (width * height * bit-depth) / 8 + 3 * num_colors • (4) divide by 1,000 or 1,000,000 to convert to kilobytes or megabytes

  17. Other Image Formats JPEG: wellesley.jpg - millions of colors, compressed, nice for photos, does not allow transparency PNG: cs110-logo.png - millions of colors, compressed, nice for line art and graphics, allows 8-bit transparency GIF: palette.gif - limited to 256 colors, nice for line art and graphics, allows 1-bit transparency, supports animation Generate all formats and compare with Pixlr!

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